The present invention relates to a method for protecting a hands-free starting system for motor vehicles. The invention concerns vehicles equipped with an authorization system for starting the engine of the vehicle, generally associated with a system of hands-free access, i.e. of detection of an electronic key for the locking or unlocking of openings.
For these two systems, information is transmitted between one or more emitters/receivers located in the vehicle and the electronic key carried by the user of the vehicle. With this aim, the vehicles are provided with items of electronic equipment intended to bring to the driver user-friendliness and security, such as in particular equipment for locking and unlocking the vehicle, and for starting it.
In this equipment, transmission is carried out by radio frequency signals, in the low-frequency region, 125 kHz for example, for their emission from the vehicle, and in an authorized radio frequency region, for example at 433 MHz or 315 MHz, for their emission from the electronic key.
The use of a physical key for locking or unlocking openings and for starting has thus been replaced by remote unlocking and, for starting, pressing for example on a button, the key remaining in the pocket or bag of the driver, close to said button.
These hands-free access and starting systems are named, for example, PASE (Passive Access and Start Engine) or PKES (Passive Keyless Entry and Start) systems. These hands-free systems are particularly advantageous because the driver no longer has to search through his pocket or bag to find the key.
However, using radio frequency communications instead of a physical key presents a security problem: specifically, this use can allow the interception of the radio frequency communications by third-party electronic equipment. To mitigate this risk, provision has been made to secure exchanges of information in the locking/unlocking and starting authorization protocols.
More precisely, the usual protocol of a hands-free access and starting system comprises the following steps:                a central control unit on board the vehicle remains on standby awaiting the detection of an unlocking request, which can take the form either of a hand approaching a capacitive sensor situated in the handle of a door, or pressing a door handle or key button;        following the unlocking request, the emitter of the vehicle (situated in the passenger compartment and/or in the door handles) sends a coded identification request to the key in response; these messages are sent at low frequency (called LF) at 125 kHz and have a short range, from about 1.5 to 2 meters;        if one of the electronic keys of the vehicle is in the perimeter of this range, it emits its identification in the radio frequency band (for example 433 MHz or 315 MHz) with a range between about 30 and 120 meters;        if the response of the electronic key is recognized as genuine by the vehicle, the latter authorizes the unlocking of the vehicle;        the doors unlock without any other physical contact;        if the user of the vehicle is seated inside his or her vehicle and wishes to start it, the central control unit then detects, for example, a press on the starting button, then another exchange of information is engaged with the electronic key and the latter must be authenticated again. If these tests are successfully passed, the vehicle starts.        
This hands-free access and starting protocol is illustrated in FIGS. 1a and 1b. FIG. 1a represents the case of standard operation, and FIG. 1b a case of hands-free access and starting with interception by a third-party item of electronic equipment. In these two figures, the exchanges of information between the vehicle and its key or one of its electronic keys are transmitted by signals between two timescales “t”, one for the vehicle denoted “C”, and the other for the electronic key “K”, the time “t” running in the direction of the arrows, from top to bottom in the example.
The exchange of messages begins with the emission of a coded identification signal 5 by the vehicle C for the attention of the electronic key K, following the detection of the approach of a hand or of a press of a door handle or starting button. The coded identification signal 5 contains a coded identification request. The electronic key K supplies a response to this request with the same code using a message contained in a coded response signal 7 and received by the vehicle C.
However, the acknowledgement of the coded request by the electronic key K can be deferred, and a deferred response signal 9 is then transmitted after a time lag Δtk with respect to the transmission of an undeferred coded response signal 7.
This delay Δtk of acknowledgment can be due, in particular:                to a slow increase in the amplitude of the coded identification signal, due to a quality factor calibrated to allow a long range of said signal, or        to a combination of a minimum threshold of variable detection of the signal received by the electronic key and the sinusoidal nature of this signal, implying variations in the measurement of the intensity of the received signal (cf. FIGS. 2a and 2b for a more detailed description).        
For the vehicle C, the duration encompassing the transmission of the coded request and the response, also coded, of the electronic key (corresponding to the durations of transmission of the signals of identification 5 and response 7) is denoted t1. If the electronic key K responds with a time lag Δtk (with the deferred response signal 9), the overall duration t1 is also delayed and takes a value t1+αt1 (with a duration of delay Δt1 substantially equal to Δtk).
FIG. 1b repeats the exchange of messages in FIG. 1a but in the case of interception by a third-party item of electronic equipment. The coded identification signal is then received with a delay D. The transmission of this delayed identification signal 5′ is therefore “tilted” between the timescales of the vehicle C and the electronic key K due to this delay D, compared to the transmission of the non-delayed identification signal 5. The coded identification request is then delayed by the same duration D upon its reception by the electronic key. And the exchange encompassing the transmission of the delayed identification signal 5′ and of the equally delayed response signal 7′ has a duration t2 substantially longer than the duration t1, of about 8 to 32 milliseconds in the example.
The duration t1 is the reference duration validated by the manufacturer and a difference between the durations t1 and t2 signifies the presence of an interception by a third-party item of electronic equipment. This stems from the fact that the interception equipment generally includes resonant or equivalent electrical circuits that disturb transmission and induce delays in the reception of the information emitted by the vehicle for the attention of the electronic key.
This interception is then validated if the difference t2−t1 is above a preset duration, equal to 50 milliseconds in this example. However, this duration t2 can be close to the duration t1+Δt1 shown above (FIG. 1a), which corresponds to the electronic key not immediately acknowledging the coded identification request, as shown previously. And the excessive closeness of these two durations, t1+Δt1 and t2, can then give rise to an interpretation error on the part of the vehicle. This example shows that the time lag (duration Δtk) contributed by the electronic key K when acknowledging the identification request in the absence of any interception can be equivalent to the delay D contributed by the interception performed by a third-party item of electronic equipment.
This situation impedes the making of decisions at vehicle level concerning the possible presence of an item of electronic equipment intercepting the signal. It is therefore important to be able to differentiate between the delay Δtk due to the electronic key and the delay D provoked by the intercepting equipment.
In these conditions, wireless communications between the vehicle and an electronic key can therefore be intercepted easily. At the present time, various solutions have been proposed to protect the vehicle from an undesirable interception. The patent document FR 2 933 437 proposes to set up detection zones around the vehicle to authorize first, out of a concern for security, the lateral unlocking of a door or of a group of doors only on the side where the fob that has been authenticated by the vehicle is found.
Moreover, the use of codes regularly renewed using code generating equipment is now widespread. Such a system is described in the patent document U.S. Pat. No. 6,870,459 for example.
But these solutions do not make it possible to reliably detect the interception of communications between a key and a vehicle by a third-party item of equipment.